During sea navigation satellite communications allow to receive easily TV transmissions broadcast by many satellites.
A TV satellite antenna to be mounted on a watercraft normally comprises a parabolic dish and a “feed”, i.e. a device that receives the signal reflected by the parabolic dish and transmits it to the TV decoder through a co-axial cable. In many antennas the feed consists of a device arranged at the focus of the parabolic dish and called LNB (Low Noise Block), where a reduction of the frequency for reducing the noise is carried out. Then, the signal reaches the TV decoder through a co-axial cable at a much lower frequency and easily transportable with limited losses.
At the wavelengths normally used in TV satellite transmissions, a parabolic dish with double reflection feed is also used, which comprises a reflecting plate, or mirror, which directs the signal already reflected by the parabolic dish towards a tubular wave guide, co-axial to the parabolic dish. The tubular wave-guide directs the signal towards an LNB converter and then to the TV decoder. The LNB converter is arranged behind the parabolic dish, and not in the focus of the parabolic dish, with the advantage of eliminating thus all the noise made by the circuits of the converter same
Recently, some TV satellites have been equipped with transponders capable of assuring transmissions for allowing Internet surfing. For example, the satellites EUTELSAT and ASTRA in addition to broadcasting many TV channels, also give access to Internet. In fact, using a special electronic board a computer on a watercraft can download data (downlink) at a speed presently of 2 MBit/s. In this case signals directed to the satellite (uplink) are sent through a portable satellite telephone (or other system of communication towards satellite) at a much lower speed. Such system is called “unbalanced”, owing to the large difference between the speeds of uplink and downlink.
In order to receive and transmit data via Internet in a “balanced” bidirectional way, it is therefore necessary, according to the present technique, a second transceiving antenna satellite. This causes higher costs and also problems of space on the watercrafts.
Alternatively, it is possible to use a satellite telephone, with increase of costs and low speed of data transmission.
Bringing on a same antenna a double TV/Internet communication causes, on the other hand, some technical problems. In fact, the TV satellite channels normally transmit on a band of about 12 GHz (KU-band: 10.7-12.7 GHz), whereas Internet communications are exchanged presently in L-band (about 1500-1600 MHz). Owing to the large difference of frequency, it is not possible with the present techniques use on a parabolic dish a same feed device.